1. Field of Invention
The present invention relates to a non-aqueous electrolytic solution and an electrochemical energy storage device comprising the same. More particularly, this invention pertains to non-aqueous electrolytic solutions that comprise (a) one or more solvents; (b) one or more ionic salts; and (c) one or more additives. Non-aqueous electrolytic solutions capable of preventing fire or reducing flammability in energy storage electrochemical cells (e.g., lithium metal batteries, lithium ion batteries, lithium ion capacitors and supercapacitors) include a non-fluorinated carbonate, a lithium salt, a phosphazene compound, and a fluorinated solvent. Such electrolytic solutions enhance the battery performance, reduce capacity degradation during cycling and high temperature storage, and prevent or reduce flammability of a battery made therewith.
2. Description of Related Art
Safety is a primary concern for lithium ion batteries, especially large batteries with applications in xEVs (plug-in vehicles). The conventional electrolytic solutions currently used in lithium or lithium ion batteries are highly flammable, and there is justifiable concern that in an accident situation a potential exists for leakage from the batteries, which would thus burn in the presence of flame or spark. There is a need to improve the safety of the battery system by finding an electrolytic solution that is non-flammable, diminishing the chances that when the container is compromised in an accident the cells would easily catch fire.
Fluorinated phosphazene compounds have shown excellent flame retarding properties. However, for highly flammable solvents, such as EMC, in the electrolytic solution, a high concentration of additive(s) is required to achieve non-flammability. The high concentration required may cause a decrease in conductivity and can sometimes lead to deterioration in cell performance. Another concern with such compounds is the cost: they are about 10 times more expensive than other materials used in the electrolytic solution.
Fluorinated carbonates, fluorinated ethers, fluorinated esters, fluorinated alkanes, fluorinated alkyl phosphates, fluorinated alkyl phosphonate, and fluorinated aromatic phosphonate are other important nonflammable solvents. However, very high concentrations of these compounds (30% or higher) in the electrolytic solution is typically required to achieve the non-flammability desired. This high concentration of materials may cause problems such as poor salt solubility, phase separation of the electrolytic solution itself, and the potential for loss of cell performance.
Because of the high costs and possibility of poor cell performance associated with high concentrations of conventional flame retardants, it is imperative to find alternative formulations and materials that may improve upon either or both of these problems.